FIG. 6 is a cross-sectional view illustrating a prior art buried heterostructure (BH) laser diode which is illustrated in, for example, SPIE-The International Society for Optical Engineering, Volume 2148, pages 142-151.
In the figure, reference numeral 1 designates a semiconductor substrate comprising p type InP. Reference numeral 3 designates a p type InP cladding layer including a buffer layer 2. An active layer 4 comprising InGaAsP is disposed on the p type InP cladding layer 3. A first n type cladding layer 5 comprising n type InP is disposed on the active layer 4. P type current blocking layers comprising p type InP are located on both sides of a mesa comprising the lower cladding layer 3, the active layer 4, and the upper cladding layer 5. An n type current blocking layer 7 comprises n type InP. A second n type cladding layer 8 comprising n type InP is in contact with the upper surface of the p type current blocking layer 6. A contact layer 9 comprising N type InP for making an ohmic contact is disposed on the second n type cladding layer 8. Reference numerals 11, 12, 13, and 14 designate crystalline regrowth interfaces, and reference numeral 10 designates an electrode comprising a metal.
In the figure, each arrow represents a current. The crystalline regrowth interfaces 11, 12, 13, and 14 that are formed during crystalline growth steps, regions A, B, C, and D, represented by circles, are regions that deteriorate during high temperature operation and through which leakage currents represented by arrows flow without flowing through the active layer 4. Deterioration is likely to occur at the pn junction interfaces between p type InP and n type InP, i.e., at the crystalline regrowth interfaces 11, 12, 13, and 14, during successive crystalline growth steps. Generally, the crystalline regrowth interfaces 11, 12, 13, and 14 are likely to be exposed to air or etched, resulting in the formation of natural oxide films or the presence of impurities, producing many crystalline defects.
FIGS. 7(a)-7(d) are cross sectional views for explaining the process of fabricating the BH laser. As shown in FIG. 7(a), a p type cladding layer 3 comprising p type InP including a buffer layer 2, an active layer 4 comprising InGaAsP, and a first n type cladding layer 5 comprising n type InP are successively grown by MOCVD (metal organic chemical vapor deposition). Then a silicon dioxide film is grown, and the silicon dioxide film is patterned by photolithography to form a mask 15.
Subsequently, a shaped stripe mesa 16 is formed by etching, as shown in FIG. 7(b). The side wall of the stripe structure 16 is exposed to the etching material as well as to the air, whereby impurities and a natural oxide film are present at the side wall of the stripe structure 16.
Next, as shown in FIG. 7(c), a p type current blocking layer 6 comprising p type InP, an n type current blocking layer 7 comprising InP, and a p type current blocking layer 6 comprising p type InP are successively grown by MOCVD, thereby burying the active layer 4. Thereafter, the mask 15 is removed by etching. Then, on the surface of the p type current blocking layer 6 etching material remains as residual impurities along with a natural oxide film.
Subsequently, as shown in FIG. 7(d), a second n type cladding layer 8 comprising n type InP and a contact layer 9 comprising n type InP are grown by MOCVD and, thereafter, an electrode 10 is formed.
As is apparent from this fabricating process, there are two crystalline growth processes and there are impurities and natural oxide films at the regions A, B, C, and D in FIG. 6, i.e., at the pn junction interfaces of the crystalline regrowth interfaces 11, 12, 13, and 14 between the p type current blocking layer 6 and the first n type cladding layer 5, and between the p type current blocking layer 6 and the second n type cladding layer 8, whereby leakage currents that are ineffective in producing laser oscillation flow in the regions A, B, C, and D, thereby increasing operation current. This results in deterioration in the laser characteristics and deteriorated reliability.
While the foregoing description is concerned with a BH laser structure employing a semiconductor substrate comprising p type InP, in any of the BC (buried crescent) laser structure of FIG. 8 (described in Extended Abstracts of the 15th Conference on Solid State Devices and Materials, Tokyo, 1983, pp.337-340), the BH laser structure employing a semiconductor substrate comprising n type inP shown in FIG. 9, the BR (buried ridge)laser structure employing a semiconductor substrate comprising n type GaAs shown in FIG. 10, there arises laser deterioration at the regrown pn junction interfaces.
The BC laser structure shown in FIG. 8 includes a semiconductor substrate 1 comprising p type InP, an n type current blocking layer 18 comprising n type InP, a p type cladding layer 19 comprising p type InP, an active layer comprising InGaAsP, a first n type cladding layer 21 comprising n type InP, a second n type cladding layer 22 comprising n type InP, a contact layer 23 comprising n type InP, and an electrode 10. This BC laser is fabricated by successively growing on a p type InP semiconductor substrate 1 an n type current blocking layer 17 and a p type current blocking layer 18, successively, forming a stripe shaped groove on the p type current blocking layer 18 having a depth reaching the p type InP semiconductor substrate 1 from the surface of the p type current blocking layer 18, removing the mask, forming a p type InP cladding layer 19, an InGaAsP active layer 20, a first n type cladding layer 21 by liquid phase epitaxy so as to fill the groove, and successively forming a second n type cladding layer 22 and an n type InP contact layer 23, and an electrode 10. During these processes, there are formed the crystalline regrowth interface 24 between the n type current blocking layer 17 and the p type cladding layer 19, and the crystalline regrowth interface 25 between the p type current blocking layer 18 and the first n type cladding layer 21 and second n type cladding layer 22, at which the laser deteriorations would occur.
The BH laser structure shown in FIG. 9 includes an n type InP semiconductor substrate 26, an n type cladding layer 28 comprising n type InP including a buffer layer 27, an active layer 4 comprising InGaAsP, a first p type cladding layer 29 comprising p type InP, a p type current blocking layer 30 comprising p type InP, an n type current blocking layer 31 comprising n type InP, and a second p type cladding layer 32 comprising p type InP and a contact layer 33 comprising p type Inp. This BH laser is fabricated by growing on an n type InP semiconductor substrate 26 an n type cladding layer 28 including an n type InP buffer layer 27, an InGaAsP active layer 4, a first p type InP cladding layer 29 successively by MOCVD, then forming a mesa shaped stripe structure by etching from the surface of p type InP cladding layer 29 to reach the semiconductor substrate 26, employing a stripe shaped insulating film mask (not shown), regrowing the p type InP current blocking layer 30 and the n type InP current blocking layer 31 by MOCVD so as to bury the mesa shaped stripe, employing the mask as a selective growth mask, then removing the mask, and regrowing the second p type InP cladding layer 32 and p type InP contact layer 33. During these processes, laser deterioration occurs at the pn junctions at the crystalline regrowth interface 34 between the n type cladding layer 28 and the p type current blocking layer 30 and at the crystalline regrowth interface 35 between the n type current blocking layer 31 and the p type cladding layer 29 and second p type cladding layer 32.
In addition, the BR laser structure shown in FIG. 10 includes a semiconductor substrate 36 comprising n type GaAs, an n type cladding layer 37 comprising n type AlGaInP, a first p type cladding layer 38 comprising p type GaAs, an active layer 39 comprising GaAsP, an n type current blocking layer 40 comprising n type GaAs, a second p type cladding layer 41 and a contact layer 42 both comprising p type GaAs, and an electrode 10. This BR laser is fabricated by successively growing on an n type GaAs semiconductor substrate 36 an n type AIGaInP cladding layer 37, a GaAsP active layer 39, and a first p type GaAs cladding layer 38 by MOCVD, forming a stripe shaped mask (not shown) comprising an insulating film, etching to the first p type GaAs cladding layer 38 to a depth not reaching the GaAsP active layer 39 thereby to form a ridge, regrowing the n type GaAs current blocking layer 40 by MOCVD to bury the ridge employing the mask as a selective growth mask, and, after removing the mask, successively regrowing the second p type cladding layer 41 and contact layer 42, both comprising p type GaAs employing MOCVD. During this process, laser deterioration occurs at the regrown pn junction interface 43 between the first p type cladding layer 38 and the n type current blocking layer 40 and of the crystalline regrowth interface 44 between the second p type cladding layer 41 and the n type current blocking layer 40.
As discussed above, in the prior art BH, BC, and BR lasers, there are impurities and natural oxides films at regrown pn junction interfaces. Accordingly, as the operation current increases, the idle current increases. With this idle current flowing in a high temperature condition, the deterioration of regrown pn junction interfaces is accelerated, thereby causing a change in the operation current, resulting in laser deterioration, which in turn results in deteriorated reliability.